Fiber-based Particle Position Detection

Fiber-based position detection of trapped particles can be used to calibrate the spring constant of the optical trap. According to our previous work^{36 (link)}, the differential signals (scattering light) collected by two fibers in AFMOTs scale with y-displacement (Fig. 1c) relative to the trap center when the motion of the trapped particle is small (< 100 nm). Once a particle is trapped, no objective lens or microscope is required to detect the particle position. In the experiment, the back-scattered light by the trapped particle was collected by the two prefixed optical fibers and measured by two inputs (PD1 and PD2) of a balanced photodiode (PDB450C, THORLABS). AFMOTs can achieve 2 nm spatial resolution and hundreds of MHz bandwidth.